During the filling of sacks it is necessary to fill them with a certain filling weight in order to obtain sacks with a uniform weight which can be sold by the piece.
In an apparatus of the kind described in EP-A-290 879, the sacks are filled via a filling neck with a predetermined quantity of filling material being previously weighed out, so that due to the filling of the sacks with a net filling amount, a weighing of the sacks suspended from a pair of grippers is not necessary during filling. This method of filling sacks with a weighed out net filling amount is, however, impossible when the sacks are filled with filling material of a poor flowing behavior, e.g. with liquid filling material of a high viscosity, since such filling materials tend to adhere to the filling neck or to the guide means used for the filling, so that when a net filling amount is fed via the filling neck it is not clear whether the complete quantity of filling material will be received entirely by the sack or bag to be filled.
If the filling of sacks or bags with a previously weighed net filling quantity is impossible, a so-called gross weighing must be carried out, which means that the sacks are weighed during their filling with the filling material and the sacks are filled until the weighing indicates the desired amount of filling. This kind of gross weighing includes the problem that not just the sack along, together with the filling amount received by the sack is weighed, but that also the means holding the sack during filling must be weighed together with the sack and the filling quantity. The weight of the means holding the sack enters into the gross weighing as dead weight so that, when taking into account this dead weight, even for small relative errors (inaccuracies) in the measuring result of the gross weighing, relatively large deviations from the desired net weight to be filled into the sacks may be obtained.
From DE-PS 499 716 as well as from U.S. Pat. No. 2,546,193, filling apparatus for sacks with a gross weighing of the filling quantity are known, in which, however, in addition to the holding means for the sacks to be filled, the entire filling hopper arrangement is included in the dead weight weighed therewith. For that reason it is impossible in these known apparatus to keep the inevitable relative measuring error so small during weighing that the absolute errors of the desired filling amount can be kept in the desired small and, therefore, negligible range.